Summary: in this tutorial, you will learn how to use the PostgreSQL INTERSECT operators to combine two or more SELECT statements to form a single result set.
Introduction to PostgreSQL INTERSECT operator
Like the UNION and EXCEPT operators, the PostgreSQL INTERSECT operator combines the result sets of two or more SELECT statements into a single result set. The INTERSECT operator returns all rows in the both result sets.
The following illustration shows the final result set produced by the INTERSECT operator. The final result set is represented by the yellow area where the circle A intersects with the circle B.
The following illustrates the syntax of the INTERSECT operator:
1 2 3 4 5 6 7 8 9 | SELECT column_list FROM A INTERSECT SELECT column_list FROM B; |
To use the INTERSECT operator, the columns that appear in the SELECT statements must follow the rules below:
- The number of columns and their order in the
SELECTclauses must the be the same. - The data types of the columns must be compatible.
PostgreSQL INTERSECT operator example
Let’s create some tables for the demonstration.
The following CREATE TABLE statement create three tables: employees , keys, and hipos.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | CREATE TABLE employees ( employee_id serial PRIMARY KEY, employee_name VARCHAR (255) NOT NULL ); CREATE TABLE keys ( employee_id INT PRIMARY KEY, effective_date DATE NOT NULL, FOREIGN KEY (employee_id) REFERENCES employees (employee_id) ); CREATE TABLE hipos ( employee_id INT PRIMARY KEY, effective_date DATE NOT NULL, FOREIGN KEY (employee_id) REFERENCES employees (employee_id) ); |
The employees table stores the employee master data. The keys table stores the key employees and the hipos table stores the employees who are high potential and high impact.
The following INSERT statements insert some sample data into the employees, keys, and hipos tables:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | INSERT INTO employees (employee_name) VALUES ('Joyce Edwards'), ('Diane Collins'), ('Alice Stewart'), ('Julie Sanchez'), ('Heather Morris'), ('Teresa Rogers'), ('Doris Reed'), ('Gloria Cook'), ('Evelyn Morgan'), ('Jean Bell'); INSERT INTO keys VALUES (1, '2000-02-01'), (2, '2001-06-01'), (5, '2002-01-01'), (7, '2005-06-01'); INSERT INTO hipos VALUES (9, '2000-01-01'), (2, '2002-06-01'), (5, '2006-06-01'), (10, '2005-06-01'); |
The following statement returns the key employees from the keys table.
1 2 3 4 | SELECT employee_id FROM keys; |
1 2 3 4 5 6 7 | employee_id ------------- 1 2 5 7 (4 rows) |
The following statement returns a list of employee_id from the hipos table.
1 2 3 | SELECT employee_id FROM hipos; |
1 2 3 4 5 6 7 | employee_id ------------- 9 2 5 10 (4 rows) |
To get the employees who are both key, and high potential and high impact.
1 2 3 4 5 6 7 8 9 | SELECT employee_id FROM keys INTERSECT SELECT employee_id FROM hipos; |
1 2 3 4 5 | employee_id ------------- 5 2 (2 rows) |
The result set shows that the employee id 2 and 5 are the are key, high potential, and high impact employees.
To sort the result set returned by the INTERSECT operator, you place the ORDER BY clause at the end of the statement, not at the end of each SELECT statement.
See the following query:
1 2 3 4 5 6 7 8 9 10 | SELECT employee_id FROM keys INTERSECT SELECT employee_id FROM hipos ORDER BY employee_id; |
1 2 3 4 5 | employee_id ------------- 2 5 (2 rows) |
In this tutorial, we have shown you how to use the PostgreSQL INTERSECT operator to combine multiple result sets and return the rows that appear in both result sets.